1. Field of the Invention
The present invention relates to an environment recognition device and an environment recognition method for recognizing a target object based on a luminance of the target object in a detection area.
2. Description of Related Art
Conventionally, a technique has been known that detects a target object such as an obstacle including a vehicle and a traffic light located in front of a subject vehicle for performing control to avoid collision with the detected target object and to maintain a safe distance between the subject vehicle and the preceding vehicle (for example, Japanese Patent No. 3349060 (Japanese Patent Application Laid-Open (JP-A) No. 10-283461)).
Further, in such techniques, there is a technique that specifies a target object uniformly as a solid object, but further determines whether the detected target object is a preceding vehicle that is running at the same speed as the subject vehicle or a fixed object that does not move. In this case, when the target object is detected by capturing an image of a detection area, it is necessary to extract (cut out) the target object from the captured image before specifying what the target object is.
For example, there is known a technique that extracts a pixel (edge pixel) having an edge from a captured image based on a derivative value between adjacent pixels, derives a histogram (distance distribution) of the edge pixel in the width direction and in the height direction of the image, and estimates a region corresponding to its peak as an edge of a target object. There is also disclosed a technique for determining whether a target object is a vehicle through a comparison between a fusion pattern based on a histogram and a dictionary pattern stored beforehand (e.g., JP-A No. 2003-99762).
However, in the conventional method of deriving the histogram in the horizontal direction and in the vertical direction, a preceding vehicle ahead of a subject vehicle is imaged relatively on a tilt, and its edge does not exactly extend in the width direction and in the height direction, when a road surface is inclined with respect to the advancing direction of the subject vehicle. Therefore, there is no peak in the histogram in the width direction and in the height direction, resulting in that the accuracy of detecting a vehicle is deteriorated. Similarly, the peak of the histogram in the width direction and in the height direction is difficult to appear in a vehicle having many curved surfaces, resulting in that such vehicle is difficult to be specified.